Various techniques for symbol recovery are known but most are complex in terms of structure or operation, often requiring extensive circuitry or processing resources to accomplish there tasks. This is due in part to the many ways a base band signal that needs to be transmitted can be corrupted prior to being presented once again as a base band signal to the apparatus responsible for symbol recovery. This corruption can result from properties of the transmitters and receivers as well phenomenon that occur or impact the transmission channel. For example there will be frequency differences and timing differences between the transmitters and receivers. Both will have some degree of filtering that can have an adverse impact on the received signal. Additionally the channel will be subject to various known forms of interference including multipath conditions. In the end the signal processors responsible for symbol recovery will have to account for each of these possible problems and this can result in very complex arrangements which are energy intensive. This is a problem for small probably portable battery powered equipment. Additionally this complexity normally is costly in economic terms so very cost sensitive applications suffer.
One known approach for symbol recovery is a maximum likelihood sequence estimator (MLSE) which essentially attempts to compare the received signal to that which would be expected for possible sequences of symbols. This has been found to be useful when a signal is subject to inter symbol interference (ISI) sometimes also known as delay spread or symbol dispersion. Generally this is a known phenomenon where one symbol will impact or interfere with or corrupt symbols in subsequent symbol periods. Usually some probability is assessed for each sequence and one is eventually selected based on this probability or likelihood. Having selected the sequence the symbols will be recovered as the ones corresponding to the selected sequence. These operations of determining probabilities, selecting sequences and thus symbols is often accomplished very effectively with a technique normally referred to as a Viterbi decision process or search. While effective the processing capacity or circuit complexity for accomplishing a Viterbi search can be excessive for economically sensitive applications particularly when long data packets must be processed. Additionally this approach will introduce a latency in symbol recovery that can be excessive. This results from having to wait until a sequence is selected before the first symbol is selected. While techniques are known for helping with these complexity and latency concerns none are known that are effective enough for particularly sensitive applications.
What is needed is a low cost low power elegant method and apparatus for recovering symbols in a system where the symbols are subject to ISI.